1. Field of the Invention
The present invention relates to a linear motor which includes a stator having a drive field magnet, i.e., field magnet for a driving purpose, extending in a uniform direction and a movable piece having an armature coil opposed to the field magnet, the stator being carried at its predetermined portion by a carrying member.
2. Description of the Background Art
Linear motors, each of which includes a stator having a drive field magnet extending in a uniform direction and a movable piece having an armature coil opposed to the field magnet, are disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 61-9161 (1986) and 58-36162 (1983).
The linear motor taught by Japanese Laid-Open Patent Publication No. 61-9161 is shown in FIGS. 7(A) and 7(B). This linear motor includes a plate-like stator 1 having a field magnet 11, which is provided at its surface with N- and S-poles arranged in a uniform direction, and a movable piece 2 having an armature coil 21 which is opposed to the field magnet 11.
The field magnet 11 in the stator 1 is carried by a stator yoke 12, which is mounted on a base 13.
The armature coil 21 of the movable piece 2 is formed of a plurality of air-core coils 21a, which have air-cores opposed to the field magnet 11 and are parallel to the field magnet 11, and are carried by a movable piece yoke 22. The movable piece yoke 22 has its opposite side ends which are bent downward and carry guide rollers 23. The guide rollers 23 are guided by the side surfaces of the stator yoke 12 for movement on the base 13.
As shown in FIG. 7(B), the armature coil 21 includes a position sensing element 24, which is operable to sense a magnetic force of the field magnet 11 at the position for controlling current supply to the coil, and a magnetic sensor 25 of an encoder provided for control of the speed of the movable piece and others. The position sensing element 24 is opposed to one of the side surfaces of the field magnet 11, and the magnetic sensor 25 is opposed to a fine magnet row 14 (i.e., a row of fine magnetized portions) which is formed at the other side surface of the field magnet 11. The sensors 24 and 25 are movable together with the movable piece 2 for sensing magnetism.
Meanwhile, the linear motor taught by Japanese Laid-open Patent Publication No. 58-36162 includes, as shown in FIG. 7(C), a rod-like stator 3 having a field magnet 31, which is provided at its surface with N- and S-poles arranged alternately in a uniform direction, and a cylindrical movable piece 30 fitted around the stator 3 and having an armature coil 301.
The armature coil 301 is formed of ring-like coils 301a fitted around the field magnet 31, and is attached to an inner side of a cylindrical movable piece yoke 302. In the example shown in FIG. 7(C), the cylindrical movable piece yoke 302 further includes bearings 303 fitted to opposite ends of the yoke 302, so that the movable piece 30 can move relatively along the stator 3. Various types of bearings other than the above bearings may be employed. The above publication discloses a structure, in which a plastic projection formed at the inner surface of the armature coil is slidably fitted into a longitudinal groove formed at the field magnet.
For the purpose of control of current supply to the armature coil 301, the movable piece 30 is provided with position sensing elements 24 such as a hall element for sensing a magnetic force of the field magnet at the position.
In the linear motor of the above type which includes the rod-like stator having the drive field magnet and extending in a uniform direction and the movable piece having the armature coil, which is fitted around the field magnet and is arranged inside the cylindrical movable piece yoke, the rod-like stator itself can be used as a guide member for the movable piece, and it is not necessary to add large or complicated guide means for the movable piece, so that the structure can be simple, and the operation accuracy is high. By these and other reasons, the linear motors of the above type have been employed in various equipments and instruments. For example, it has been attempted to use the above linear motor as a unit, which drives an optical scanning system for an image of a document or original in an image forming device such as a copying machine and an image reading device such as an image scanner.
As described above, the linear motor, which includes the rod-like stator having the drive field magnet and extending in a uniform direction and the movable piece having the armature coil fitted around the field magnet and arranged inside the cylindrical movable piece yoke, is utilized, for example, as a unit for driving the optical scanning system for a document image in the above image reading device. In this case, the movable piece is connected to a carriage, which carry a lighting lamp for a document image and/or a light reflecting mirrors, and the rod-like stator is usually carried at its opposite ends by carrying members, e.g., each made of a metal plate. Each region including the portion carried by the carrying member is provided with magnetic poles such as a field magnet for driving the movable piece.
As described above, the stator is provided even at the portions carried by the carrying members with the magnetic poles. Therefore, in a wide range including the portions which are in contact with the carrying members, the magnetic field of the drive field magnet or the like of the stator is increased or weaken by the influence of the carrying members. If the magnetic field of the field magnet causing a large magnetic force increases, torque ripple (so-called cogging) of the movable piece occurs. If it decreases, a sufficient force for driving the movable piece of the piece cannot be obtained, and thus the rapid increase and decrease of the speed cannot be performed. In the structure where the stator is provided with the fine magnet row for reading by the encoder, as employed in the linear motor shown in FIG. 7(B), an error occurs in its magnetism detection.
The above problem also arises in the following case. As exemplified in FIG. 7(A), the linear motor includes the plate-like stator having the field magnet extending in a uniform direction, the armature coil of the movable piece is arranged oppositely to the stator for driving the movable piece, and the stator is partially carried by the carrying member which exerts an influence on the magnetic field.